Yeah, those tank prices are a little high. Have you tried asking at your local welding supply house? They usually give a better deal... and are intrigued when you tell them what it's for.
Are you going to use a reactor or diffuser? That's a potential cost if you no longer want to use the Hagen ladder. Reactors can be built for reasonable cost.
If you're trying to save money, look into buying a regulator that has the works: bubble counter, solenoid, etc. Those should be under $100.